I hate labeling my post PSA, but for most visibility I might as well.

/spectate playername - puts you in a "queue" to spectate a certain player.

/replay matchid - only works with replays that have been spectated (?). Comp == decent chance

Spectating and Replays work the same, and it's somewhat buggy. I think it's really cool to spectate/replay games in free-movement mode, you can capture so many battles at once. I don't know why tournament vods aren't like this? Swapping character to character in first-person captures so little compared to this.

Controls:

WASD: Move horizontally
Q/E: Move vertically (Q/E is Down/Up)
Left Shift: Pan camera faster with WASD QE
Pause/Break: Pause
Page Up: Increase playback speed
Page Down: Decrease playback speed
Comma(,): Skip Back
Period(.): Skip Forward (Might not work?)
P: Removes HUD elements
X: Toggle some buggy third-person camera
(;): ?? stalls game for a frame or something. maybe taking a screenshot?

Numbers/Numpad Controls:

Numpad 0-9 (NumLock ON): Switch camera to first-person on corresponding characters
Numbers Row 0-9: If you hold SHIFT   Number, you will save a "camera hotkey" location, similar to SC.
                 You can recall said camera hotkey by pressing the same Number again. 

Example: Start in Player base. Press SHIFT 1. Pan to Enemy base. Press SHIFT 2. 
         Now when you tap 1 and 2, you will toggle your camera location between bases.

Problems:

• Can't exit replay/spectate to lobby. Best you can do is Page-Up to fast forward till the end.
• No POTG, gets stuck at the end (can Page-Up fast forward)

If anyone knows any more info about this, please let me know. I'd also like to know any other hidden / commands, like /lfg bringing up a lfg chat. Thanks to Moondawg for pointing this out for me. Hopefully they refine this and make it more accessible for everyone soon.